Lawn trimmers are either manual or powered by an electric motor or gasoline engine. Power lawn trimmers are typically hand-supported by an operator, which necessarily limits their size, weight and power. A hand-held trimmer, for example, cannot use a larger and generally heavier motor or engine because the operator can only safely handle a certain weight over a period of time. Any heavier weight will easily tire the operator and cause a potential safety hazard to him and to others. Thus, hand-held trimmers are limited in the type of work they can handle. Heavy trimming involving densely growing grass or vegetation generally requires a more powerful and heavier motor or engine and, therefore, may not be suitable for hand-held trimmers.
A shoulder harness has been used to help the operator support a heavier trimmer. However, in a situation where a trimmer has gone haywire, the harness may impede the operator from disengaging from it quickly enough to avoid any injury.
The cutting blade or filament normally associated with hand-held trimmers is usually only partially guarded from the operator. As a result, the cutting filament or blade can accidentally strike the operator or a person nearby when the operator loses control of the trimmer. Loss of control can occur due to fatigue from prolonged handling or due to the trimmer accidentally striking an obstacle.